1. Field of the Invention
The present invention relates to an infrared detector to detect infrared rays, an infrared detecting apparatus, and a method of manufacturing an infrared detector.
2. Description of Related Art
Among the infrared detectors that have been put to practical use, those detecting infrared rays in the far-infrared wavelength range (for example, a wavelength band ranging from 8 μm to 12 μm) include: infrared detectors made of a semiconductor of HgCdTe, which is a group II-VI compound; and quantum well infrared photo-detectors (QWIPs) made of a superlattice of a group III-V compound semiconductor (GaAs/AlGaAs).
The infrared detectors made of HgCdTe can accomplish high quantum efficiency, but the low melting point and the high vapor pressure of Hg make the growth of a high-quality crystal difficult. Manufacturing of HgCdTe image sensors results in an especially poor yield. In addition, it is impossible to obtain a high-quality substrate (CdTe or the like) with a large area, and such unavailability makes it difficult to manufacture image sensors with a large number of pixels.
QWIPs detect the infrared rays by a photo-current caused by a transition of electrons, which is caused by the absorption of the infrared rays, between the discrete energy levels of the electrons in the quantum well. The formation of the quantum well is accomplished by a superlattice structure of AlGaAs and GaAs, so that the matured technique of GaAs crystal-growth process makes it possible to manufacture high-quality image sensors with a large area. The electrons, however, are quantized only one-dimensionally, that is, only in the stacking direction of the superlattice. Accordingly, such image sensors are not sensitive to incident infrared rays from a direction that is perpendicular to the plane, which results in extremely low quantum efficiency.
Various propositions have been made thus far to address the above-mentioned problem. For example, Japanese Unexamined Patent Application Publication No. Hei 2-192769 discloses a structure in which sloping faces are formed in the substrate and a quantum-well layer is formed on the sloping faces. Japanese Patent Application Publication No. 2003-218366 discloses a lateral-direction conduction structure with self-assembled quantum dots. Japanese Patent Application Publication No. 2000-275692 discloses a wavelength-conversion structure in which the electrons generated in the QWIP are recombined in a quantum-well layer to emit near-infrared rays with a higher energy level than those of the incident infrared rays. The QWIPs that have been proposed thus far have not achieved satisfactory final detection sensitivity yet.
Far-infrared wavelength-range observation in space requires detection of infrared rays of weak intensity with a wide viewing angle and high resolution. What is necessary for this purpose is a large-area (a large-pixel-number) image sensor made of a material with high quantum efficiency. It is difficult, however, to manufacture an image sensor that can satisfy the above-mentioned two requirements.